third octave decomposition

[bastien]

Hi!

I’m working about third octave decomposition with a vibration signal.

I’m using the sound and vibration toolkit.
The sample frequency of my signal is 2400 Hz.

I use the vi “1/n octave IEC decomposition” and I would like to know if I can selected the frequency range “1 to 400 Hz” in third octave decomposition? I think the results are strange in a low frequency (1 to 4 Hz)
In fact, for example, the BW of the first 1/3 octave is less of 0.3 Hz!!!!

So it’s possible to begin the third octave decomposition at 1 Hz?


Thanks for your help.

Bastien

[Kerouac]

You can change frequency when you perform third octave analysis.
>1/ Place "third-octave analysis.vi" on diagram
>2/ Double-click on it to open its front-panel
>3/ On front-panel, you can change frequency range.
or you can use the custom octave analysis.vi

MG

[bastien]

thank...

But I would like to analyse a short signal (5 to 10 second) in the frequency range [1Hz to 400Hz].

When you begin the third octave analyse at 1 Hz you must to have a vibration signal with 17.24 s minimal length in time.


this minimum time is say in the user manual of the toolkit "sound and vibration":

Filter Settling Time :


When starting or resetting the filtering operation of the fractional-octave
filters, a certain time is required before the measurements are valid.
This time is called the settling time and is related to the bandwidth of any
particular filter. The lowest frequency band has the smallest bandwidth and
defines the settling time required before you can consider the complete
fractional-octave measurement valid. In the Sound and Vibration Toolkit,.......

!!!!!!!!!!!!!!!!!!!!!!!!

settling time is defined as five divided by the bandwidth of the filter used
for the lowest frequency band.

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

..........The filter settled indicator returns TRUE as soon as all the filters are settled.




When you begin analyse at 1 Hz, the first BW is 0.29. So, 5/0.29 = 17.24 second.

My vibrations signals are too short (5 to 10 second) and I can't have more long because this signals are due to train course and it’s very fast.

I don’t know how to catch information in a low frequency (1 Hz) with a short signal.

Thank for your help


bastien